Xona Bearing Preload System

Xona Tech - 2015

Xona Rotor has implemented a proprietary variable-preload angular contact ball bearing system to its XR product line. By providing increased preload when needed and less when it isn’t, bearing life is extended.

Thrust, or axial, loading occurs in a turbocharger when the pressures acting on the turbine and compressor wheels are unbalanced. The thrust load is directed through the turbine shaft and applied to one row of balls in the bearing. To maintain contact of the unloaded row of balls to the races, preload is required. Insufficient preload can cause the unloaded row of balls to slide and skid on their respective races, generating fretting wear on the races that results in reduced life. Excessive preload, on the other hand, accelerates wear and generates heat.

The two most common methods to generate preload in a bearing are via mechanical means or by spring force. The mechanical method requires very tight axial tolerances to be held between the bearing’s inner races and opposing outer races. To maintain consistent preload, a mechanical preload system also relies on materials of equal coefficient of thermal expansion and uniform operating temperatures across the entire bearing system.

Accordingly, the extreme temperature excursions inherent to a turbocharger present a challenging operating environment for a mechanically-preloaded ball bearing system. The inner bearing race is typically press-fit to the turbine shaft and runs hotter than the outer race, which is thermally decoupled and more directly bathed in cooling oil. Even with a water-cooled bearing housing, the temperature gradient between the outer and inner bearing races can be hundreds of degrees Fahrenheit. Turbocharger applications also present a significant axial temperature gradient along the bearing’s length due to the difference in operating temperature between the turbine and compressor. These temperature differentials will not allow optimum preload to be maintained for all operating conditions . Compared to a mechanical preload system, a spring-based system can maintain a more consistent preload across a wider range of temperatures, but provides lower system stiffness.

These conventional approaches to applying preload can only be optimized for a single design point. In all other cases they are either employing insufficient or excessive preload.

Xona Rotor’s variable-preload system addresses the limitations of conventional bearing systems. It harnesses the oil pressure delivered to the bearing housing to selectively apply preload to the rotating group as needed . It compensates for the variations in preload induced by thermal expansion and can vary preload as a function of rotational speed . This patented system is exclusive to Xona Rotor . Unlike some ball bearing turbochargers, Xona Rotor XR turbochargers also use the oil to hydraulically damp shaft motions that would otherwise compromise bearing life. The oil also damps axial motions, a feature exclusive to Xona Rotor.